The present invention relates to an improvement in apparatus for scavenging the erosion zone in spark-erosive cutting plants that use a wire or strip-like electrode and a fluid scavenging medium.
In the case of spark-erosive cutting with wire or strip-like electrodes, the electrode is held between two guide heads and is guided by the latter. Particularly in the case of cutting at large angles, guide heads may be used which are oriented to the inclined position of the wire. An efficient scavenging of the erosion zone is particularly important in the case of large cutting angles because the working length of the wire in the workpiece is considerably increase. Increased guidance distances must be maintained, and the guidance distance may be further increased to avoid collision between pivotable wire guide heads and the work piece. It is therefore particularly important to maintain efficient scavenging. A suitable scavenging apparatus must also have characteristics permitting easy use for both conical and cylindrical cutting, as well as in the case of any recutting or shaving operations.
When spark-erosive cutting plants were introduced, generally the erosion zone was only scavenged by lateral jets of scavenging fluid. Practical experience with complex workpieces soon made it necessary to use more powerful scavenging means and they rapidly became widely used in the form of so-called coaxial scavenging, e.g. DE-OS No. 24 08 715. When conical cutting with moderate angular position was subsequently introduced, these scavenging approaches were largely retained, because the positioning of the guide heads for cylindrical cutting was generally retained. In order to further increase the scavenging action, there were then numerous improvements made. For example, the mouthpiece referenced in DE-OS No. 24 08 715 was given a special nozzle configuration according to DE-OS No. 28 09 506. In addition, the nozzle outlets were combined in a closed unit in the wire guide head, such as could also be used for the flow transfer to the workpiece.
It is a general disadvantage of all the aforementioned approaches that in the case of more pronounced conical cutting, there arises misalignment between the axis of the scavenging fluid and the axis of the wire electrode, due to the necessary distance between the jet outlet and the start of the cutting point--and the distance increases as the contour inclination angle increases. Thus, a larger cross-section of the coaxial jet strikes the workpiece surface on one side of the wire and is displaced in one direction. As a result, the flow of the scavenging fluid is increased, leading to the drawing in of air, which is then also drawn into the work zone. Attempts to prevent such air disturbances through the use of larger coaxial flow cross-sections or by working in a fluid bath are technically unsatisfactory solutions, because apart from the greater fluid consumption and the widely distributed secondary electrolysis, these approaches are also prejudicial with respect to idle times and splashing or spray protection. Splashing or spray protection measures became particularly necessary when increasing the scavenging of the erosion zone through the use of higher scavenging pressures, as when scavenging is done in one direction through the workpiece by supplying scavenging fluid under pressure on one side of the workpiece and removing the fluid on the other side of the workpiece by suction as described in DE-OS No. 28 33 765.
According to a further proposal (JP-OS No. 56-114628), the outlet of the coaxial scavenging nozzle can be oriented in accordance with the sloping position of the wire and positioned under numerical control by modification of the cutting vector. Although, compared with the first-described solutions, a certain improvement is obtained with regard to the quality of the erosion zone at considerable additional expenditure, the disadvantage still exists that the scavenging jet issuing in sloping manner on the wire inlet to the workpiece is once again deflected to one side, so that the risk of sucking in air sill exists.
A further proposal known from WO No. 80/02395, as is explained hereinafter relative to FIG. 1, provides oriented guide heads which are suspended on gimbals on supporting arms. Nozzle outlets for the scavenging fluid are provided on the guide heads, and the guide heads are positioned to direct a scavenging jet to the work zone. However, this approach fails to remove the difficulties of the described approaches with fixed scavenging heads in that there again, it is easier for the fluid to escape from the scavenging flow at the obtuse-angled impact point to the scavenging jet at the inlet point for wire on the workpiece than on the opposite, acute-angled side. The flow takes the path of least resistance, in that the larger quantity flows out of the abtuse-angled side, which leads to vacuum effects and the drawing in of air on the acute-angled side. Further, it is apparent that by use of the oriented guide heads, relatively large contour slope angles can be cut, but there is an increased cutting length in the workpiece and there is an increased distance betwen the guide means and the workpiece.